The present invention relates to radiation detection technology and more specifically to dosimeters that utilize thermoluminescent materials to measure radiation dosage.
Dosimeters that utilize thermoluminescent materials have been in widescale use for many years. The thermoluminescent materials employed in these devices are crystalline compounds that contain impurities and structural imperfections of various sorts such as missing atoms or ions and regions of misregistry between the planes of their crystal lattices. Some of these imperfections have the ability to capture or "trap" charge carriers in excited states of the type generated by exposure to ionizing radiation, such as gamma or beta radiation. The strength with which the charge carriers are bound by such traps depends upon the nature of the trap and its depth.
Traps are usually characterized by their thermal energy, that is, the temperature to which the thermoluminescent material must be heated in order to ensure release of the charges held by the traps. When these charges are released luminescence commonly occurs. The light quanta emitted may be photometrically detected and related to radiation exposure. Therefore, dosimeters containing thermoluminescent materials are conventionally read out by heating to about 533 K while the intensity of the light emitted by the material is recorded in the form of a "glow curve", the size of which is representative of the amount of ionizing radiation absorbed by the dosimeter.
The above-described procedures (commonly referred to as "TLD" techniques) may be perceived to have a number of important drawbacks. Only traps of intermediate energy between about 470 K and 570 K can be read out, since at temperatures above this range substantial amounts of thermal quenching occur due to non-luminescence producing transitions whereby quantum efficiencies are significantly lowered. Consequently, the information represented by a large number of high energy traps is totally ignored. Further, the incandescent radiation produced by the thermoluminescent material at temperatures above about 570 K raises the level of background noise thereby lowering the signal quality.
U.S. Pat. No. 4,954,707, the disclosure of which is hereby incorporated by reference herein, discloses a method for measuring radiation dose by use of thermoluminescent material that does not require heating of the thermoluminescent material above normal room temperature, which is about 293 K. In accordance with the method described in U.S. Pat. No. 4,954,707, the thermoluminescent material is cooled to a temperature below 200 K and is exposed to light radiation, whereby high temperature traps are converted into low temperature traps. The thermoluminescent material is allowed to warm, and the light released by the thermoluminescent material as the charge carriers are released by the low temperature traps is detected.
The thermoluminescent materials described in U.S. Pat. No. 4,954,707 that have been suitable for use in the method described therein are polycrystalline compounds doped with a metallic element, such as CaF.sub.2 :Mn.
In addition to the ability to respond to incident radiation in a manner that can subsequently be read out, it is important for realization of a practical dosimeter based on thermoluminescent material that it be possible to erase or anneal the thermoluminescent material after readout, so as to restore it to its original condition and enable reuse of the dosimeter. It is known to anneal CaF.sub.2 :Mn by heat treatment at 673 K for 15-20 minutes.
U.S. patent application Ser. No. 07/420,293, the disclosure of which is hereby incorporated by reference herein, discloses a method of annealing a thermoluminescent material in order to condition it for measuring a dose of ionizing radiation by the method described in U.S. Pat. No. 4,954,707. In accordance with the method described in U.S. patent application Ser. No. 07/420,293, the thermoluminescent material is annealed by exposing the material to light radiation.